Page 287 to 291 of a book titled "Analog Integrated Circuits" by David A johns and Ken Martin and published by John Wiley and Sons in 1997, describe an op-amp that has a differential output and a common mode feedback circuit for controlling the common mode of the potentials of the differential outputs.
A circuit with a differential output, for example an op amp, delivers its output signal as the difference between the potentials or currents on two output connections. In contrast a circuit with a single ended output delivers its output as a potential relative to a supply connection. In comparison to a circuit with a single ended output, a circuit with a differential output provides improved substrate and supply noise rejection, added dynamic range and systematic offset cancellation or reduction.
The common mode potential of the differential output, that is, the average of the potentials on the two output connections has no significance for the output signal. However, to ensure proper operation of the circuit, the common mode potential should be kept near some set potential. A common mode feedback circuit is commonly used to regulate the common mode of the differential output.
Johns and Martin disclose a common mode feedback circuit comprising two differential pairs, each containing a current source and a first and second IGFET, the gate of the first IGFET in each pair being coupled to a respective one of the connections of the differential output, the gate of the second mosfet being coupled to a reference potential. (IGFETs:
Insulating Gate Field Effect Transistors are also commonly referred to as MOSFETs: Metal Oxide Silicon Field transistors, which is taken to include FETS with any kind of gate electrodes, also poly-silicon gate electrodes). The outputs of the differential pairs are combined so that a current proportional to the difference between the reference potential and the common mode potential is produced. This current is used as a feedback to control the common mode of the differential output.
This common mode feedback circuit operates properly only as long as the potentials of the connections of the differential output are within the operating range of the differential pairs, that is, more than a IGFET threshold plus a current source saturation voltage away from a power supply potential. Typically this means that 1.2 Volt of the potential range between the power supply potentials cannot be used for the potentials at the differential output. For circuits with a low power supply range this is a significant limitation.